1. Field of the Invention
The present invention relates to a switch having a tactile (click) spring.
2. Description of Related Art
It is generally known to use a switch with a tactile (click) action for an inputting key of an electronic device such as a mobile phone. A switch with a click action can provide a tactile (click) feel to a user when a user presses the switch. Such a switch with a click action is provided with a tactile (click) spring.
A conventional switch 1a will be explained with reference to FIGS. 10 and 11. FIG. 10 is a sectional structure of a conventional switch 1a. FIG. 11 shows a characteristic of displacement against acting load of the conventional switch 1a. 
The switch 1a has, as shown in FIG. 10, a tactile (click) spring 2, switch base 3, stationary contacts 4, 5 and 6, and spring holding sheet 7. The click spring 2 is a dome-shaped spring as a contact and made of conducting metal. The click spring 2 has a circular shape in a plan view and the center of the circle is designated as a movable contact 2a. FIG. 10 is a cross-sectional view along a plane passing through the movable contact 2a of the click spring 2.
The switch base 3 is a base on which the click spring 2 is disposed and supports the spring holding sheet 7. The switch base 3 is provided with stationary contacts 4, 5 and 6. The stationary contacts 4, 5 and 6 are electrical contacts made of conducting metal. The stationary contacts 4 and 5 continuously contact and support the click spring 2. The stationary contact 6 is located at a position corresponding to the movable contact 2a of the click spring 2. The spring holding sheet 7 is adhered on the click spring 2 and fixes the position of the click spring. 2
An acting load is applied on the movable contact, within a press-down operation region R, of the click spring 2 of the switch 1a from vertically upside by a user, and a click feel is generated. The click feel felt by the user operator depends largely on characteristics of the click spring 2. In general, such a click feel can be measured by an acting load and displacement measurement device and can be shown in numeral form as an acting load to displacement curve as shown in FIG. 11, for example. A displacement (mm) indicated by a horizontal axis of FIG. 11 is a vertical displacement of the movable contact 2a of the click spring 2. An acting load (gf) indicated by a vertical axis of FIG. 11 is a vertical acting load applied to the movable contact 2a. 
As shown in FIG. 11, by applying an acting load onto the click spring 2, on which no acting load is applied at an initial state, the click spring 2 starts to deform and the displacement increases almost in a proportional relation to the increase of the acting load. By increasing of the acting load more, a click action by buckling occurs at a point of a peak acting load of F1 and a displacement of S1 and a center portion of the click spring 2 reverses to cause a displacement by an acting load smaller than the acting load F1 (acting load decreases as the displacement increases). Finally, the movable contact 2a makes in contact with the stationary contact 6 at the bottom position of an acting load F2 and displacement S2. As a result, the stationary contact 4 and 5 are brought into conduction with the stationary contact 6 through the click spring 2. When the acting load becomes zero by releasing the press down, the click spring 2 returns to the initial shape.
A click ratio is known as an indicator of the tactile feel that is defined as (acting load F1−acting load F2)/(acting load F1)×100(%). The click ratio is a variable indicating the degree of comfort of the click feel. It is also known that when a pressing position is misaligned from the center of the click spring 2 (position corresponding to movable contact 2a), an intrinsic acting load to displacement curve cannot be obtained and the click ratio may be decreased. Such a misalignment of the pressing position is caused by a tolerance of a casing, assembling misalignment or mounting misalignment on a circuit substrate, and the like. In order to suppress the decline of the click ratio caused by the pressing position misalignment, a method is known to provide a Nub (projection) on the spring holding sheet 7 (see Patent documents JP2008-269864A, JP2008-177155A, JP2006-252887A, JPH10-125172A, and JPH10-116639A, for example).
A conventional switch 1b having a Nub 8b will be explained with reference to FIGS. 12 to 15. FIG. 12 is a sectional structure of a conventional switch 1b. FIG. 13 is an acting load to displacement characteristic of the conventional switch 1b. FIG. 14 is an acting load to displacement characteristic of the conventional Nub 8b. FIG. 15 is a mechanical model of the conventional switch 1b. 
As shown in FIG. 12, the switch 1b includes a click spring 2, switch base 3, stationary contacts 4, 5 and 6, spring holding sheet 7 and Nub 8b. The Nub 8b is adhered to the spring holding sheet 7 by an adhesive 9. The Nub 8b is formed into a predetermined shape by a synthetic resin using a molding die.
The acting load to displacement characteristic of the click spring 2 is transmitted to the switch 1b via the spring holding sheet 7, the adhesive 9 and the Nub 8b in this order and measured as an acting load to displacement characteristic of the switch 1b, as shown in FIG. 13. The click spring 2 of the switch 1b, as the switch 1a, has tactile response. When the acting load F is applied on the click spring 2 of the switch 1b, the acting load increases almost proportional (linear) to the displacement and the click spring 2 buckles at the point of the acting load F1. Then a center of the click spring 2 reverses and starts displacement by an acting load smaller than the acting load F1.
As shown in FIG. 14, an acting load to displacement characteristic of the Nub 8b increases an acting load almost proportional (linear) to displacement. The gradient of the line is designated as a spring constant k1. In the same fashion, a spring constant of the spring holding sheet 7 and the Nub 8b is designated as k2. The mechanical model of such a switch 1b is described as two springs 81 and 82 that are connected in series as shown in FIG. 15. The springs 81 and 82 have spring constants k1 and k2, respectively.
As shown in FIG. 13, the acting load to displacement curve of the conventional switch 1b is linear until the acting load reaches to the peak acting load F1. Therefore, when the switch is downsized (small or low in profile), a displacement (stroke) S1 to the peak acting load F1 becomes smaller and comfortable operation (tactile) feel is not obtained.